The invention relates to an electrostatic loudspeaker comprising an electrically polarised or polarisable diaphragm arranged between two perforated, electrically conductive plates, to which an audio signal can be applied through two input terminals and comprising a frequency-response equalizing network comprising a resistor and a capacitor. Such a loudspeaker using either a diaphragm of electret foil or a diaphragm polarised by an external direct voltage for the conversion of electric audio signals into sound is known.
For a good understanding of the problems involved in the prior art and of the steps proposed by this invention to solve them first a survey of the known technology will be given.
An electrostatic loudspeaker is of the capacitative type. The capacity of a conventional, commercial system may be of the order of 2400 pF. Due to this low capacity the loudspeaker is of very high impedance. The impedance is a pure reactance which decreases with increasing frequency. In order to connect an electrostatic loudspeaker to a conventional low-ohmic amplifier a matching transformer is required. Such a transformer is usually termed a "step-up transformer", which serves to ensure that the low-ohmic amplifier finds a fairly low impedance.
Because of the frequency-dependent decrease in impedance, correct matching is in practice only possible in the range of the highest frequencies to be reproduced. It has, therefore, been proposed to split up an electrostatic loudspeaker into a plurality of systems, each of which matches the correct impedance for the highest frequencies to be reproduced thereby. By such a system reasonable efficiency can be obtained.
In practice it is found that splitting up into frequency bands brings about very serious problems because of which such relatively complicated systems do not provide the subjectively satisfying results aimed at. A solution proposed earlier, in which only one diaphragm serves to reproduce the complete frequency spectrum resides in the use of a frequency-response equalizing network comprising a parallel combination of a resistor and a capacitor, said network being included in one of the supply leads of the loudspeaker. However, the result of using such a frequency-response equalizing network is that the efficiency becomes very low. This known solution furthermore involves the great disadvantage that the absolute value of the impedance, which for all that, cannot be processed by most amplifiers without problems at the highest frequencies, drops even further. It is noted here that the absolute value of the impedance at the low-frequency system resonance in the known systems both with and without equalizing network is so high that amplifiers of some types, for example, the known very high-quality tube amplifiers, will suffer from serious mismatching, which results in deterioration of quality, whilst in addition technical problems are imminent.
Classical dynamic loudspeakers, in which, for example, a conical diaphragm is caused to vibrate by an electromagnetic driving system, are not subject to the above-described problems involved in electrostatic reproducers. However, the reproduction quality of electrostatic loudspeakers is widely considered to be appreciably better than that obtainable by electro-dynamic systems, whilst in addition electrostatic systems can provide a considerably larger dynamic range than the conventional electro-dynamic systems. The advantage of dynamic systems, however, is the materially higher efficiency.